Microwave antenna

ABSTRACT

According to one aspect of the present disclosure, a microwave antenna assembly is disclosed. The antenna assembly includes a feedline having an inner conductor, an outer conductor and an inner insulator disposed therebetween and a radiating portion including a dipole antenna having a proximal portion and a distal portion. The antenna assembly also comprises a sheath disposed over the feedline and the radiating portion defining a chamber around the feedline and the radiating portion. The chamber is adapted to circulate coolant fluid therethrough. The antenna assembly further includes a connection hub having cable connector coupled to the feedline, an inlet fluid port and an outlet fluid port. The connection hub includes a bypass tube configured to provide for flow of the coolant fluid from the cable connector directly to the outlet fluid port.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patent application Ser. No. 14/659,860, filed on Mar. 17, 2015, which is a continuation application of U.S. patent application Ser. No. 14/338,509, filed Jul. 23, 2014, now U.S. Pat. No. 9,113,932, which is a continuation application of U.S. patent application Ser. No. 14/014,937, filed Aug. 30, 2013, now U.S. Pat. No. 8,795,268, which is a continuation application of U.S. patent application Ser. No. 13/596,785, filed Aug. 28, 2012, now U.S. Pat. No. 8,523,854, which is a continuation application of U.S. patent application Ser. No. 12/199,935, filed Aug. 28, 2008, now U.S. Pat. No. 8,251,987, the entire contents of which are incorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates generally to microwave antennas used in tissue ablation procedures. More particularly, the present disclosure is directed to a microwave antenna having a coolant assembly for circulating a dielectric coolant fluid through the microwave antenna.

2. Background of Related Art

Treatment of certain diseases requires destruction of malignant tissue growths (e.g., tumors). It is known that tumor cells denature at elevated temperatures that are slightly lower than temperatures injurious to surrounding healthy cells. Therefore, known treatment methods, such as hyperthermia therapy, heat tumor cells to temperatures above 41° C., while maintaining adjacent healthy cells at lower temperatures to avoid irreversible cell damage. Such methods involve applying electromagnetic radiation to heat tissue and include ablation and coagulation of tissue. In particular, microwave energy is used to coagulate and/or ablate tissue to denature or kill the cancerous cells.

Microwave energy is applied via microwave ablation antennas that penetrate tissue to reach tumors. There are several types of microwave antennas, such as monopole and dipole, in which microwave energy radiates perpendicularly from the axis of the conductor. A monopole antenna includes a single, elongated microwave conductor whereas a dipole antenna includes two conductors. In a dipole antenna, the conductors may be in a coaxial configuration including an inner conductor and an outer conductor separated by a dielectric portion. More specifically, dipole microwave antennas may have a long, thin inner conductor that extends along a longitudinal axis of the antenna and is surrounded by an outer conductor. In certain variations, a portion or portions of the outer conductor may be selectively removed to provide more effective outward radiation of energy. This type of microwave antenna construction is typically referred to as a “leaky waveguide” or “leaky coaxial” antenna.

Conventional microwave antennas have a narrow operational bandwidth, a wavelength range at which optimal operational efficiency is achieved, and hence, are incapable of maintaining a predetermined impedance match between the microwave delivery system (e.g., generator, cable, etc.) and the tissue surrounding the microwave antenna. More specifically, as microwave energy is applied to tissue, the dielectric constant of the tissue immediately surrounding the microwave antenna decreases as the tissue is cooked. The drop causes the wavelength of the microwave energy being applied to tissue to increase beyond the bandwidth of the antenna. As a result, there is a mismatch between the bandwidth of conventional microwave antenna and the microwave energy being applied. Thus, narrow band microwave antennas may detune hindering effective energy delivery and dispersion.

SUMMARY

According to one aspect of the present disclosure, a microwave antenna assembly is disclosed. The antenna assembly includes a feedline having an inner conductor, an outer conductor and an inner insulator disposed therebetween and a radiating portion including a dipole antenna having a proximal portion and a distal portion. The antenna assembly also comprises a sheath disposed over the feedline and the radiating portion defining a chamber around the feedline and the radiating portion. The chamber is adapted to circulate coolant fluid therethrough. The antenna assembly further includes a connection hub having cable connector coupled to the feedline, an inlet fluid port and an outlet fluid port. The connection hub includes a bypass tube configured to provide for flow of the coolant fluid from the cable connector directly to the outlet fluid port.

According another aspect of the present disclosure, a microwave antenna assembly is disclosed. The antenna assembly includes a feedline having an inner conductor, an outer conductor and an inner insulator disposed therebetween and a radiating portion including a dipole antenna having a proximal portion and a distal portion. The antenna assembly also comprises a sheath disposed over the feedline and the radiating portion defining a chamber around the feedline and the radiating portion. The chamber is adapted to circulate coolant fluid therethrough. The antenna assembly further includes a three-branch connection hub including a first branch having a cable connector coupled to the feedline at a junction point, a second branch having an outlet port, a third branch having an inlet port, and a bypass tube in fluid communication with a proximal end of the first branch and the outlet port, wherein one end of the bypass tube is in proximity with the junction point to provide for flow of the coolant fluid therethrough.

A method for manufacturing a microwave antenna assembly is also contemplated by the present disclosure. The antenna assembly includes a feedline including an inner conductor, an outer conductor and an inner insulator disposed therebetween and a radiating portion including a dipole antenna having a proximal portion and a distal portion. The method includes the step of enclosing the feedline and the radiating portion in a sheath to define a chamber around the feedline and the radiating portion. The chamber is adapted to circulate coolant fluid therethrough. The method also includes the step of coupling a three-branch connection hub to the feedline and the sheath. The three-branch connection hub including a first branch having a cable connector coupled to the feedline at a junction point, a second branch having an outlet port, a third branch having an inlet port. A step of interconnecting a proximal end of the first branch and the outlet port via a bypass tube is also provided by the method. One end of the bypass tube is in proximity with the junction point to provide for flow of the coolant fluid therethrough

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the following detailed description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram of the microwave ablation system according to an embodiment of the present disclosure;

FIG. 2 is a perspective, internal view of the microwave antenna assembly according to the present disclosure;

FIGS. 3 and 4 are enlarged, cross-sectional views of a portion of the microwave antenna assembly of FIG. 1;

FIG. 5 is a side view of an interchangeable tip (or a sheath and a tip assembly) for use with the microwave antenna assembly of FIG. 1;

FIG. 6 is a schematic, top view of a connection hub of the microwave antenna assembly of FIG. 1 according to the present disclosure;

FIG. 7 a cross-sectional view of a series of inflow tubes of the microwave antenna assembly of FIG. 1 according to the present disclosure;

FIG. 8 is a topside view of a proximal portion of the microwave antenna assembly of FIG. 1 according to the present disclosure;

FIG. 9 is a side view of a proximal end of the feedline of the microwave antenna assembly of FIG. 1 according to the present disclosure;

FIG. 10 is a side view of a cable connector of the microwave antenna assembly of FIG. 1 according to the present disclosure;

FIG. 11 is a top view of a connection hub of the microwave antenna assembly of FIG. 1 according to the present disclosure;

FIGS. 12A and B are perspective and side views of the connection hub of the microwave antenna assembly of FIG. 1 according to the present disclosure;

FIG. 13 is a top view of a connection hub of the microwave antenna assembly of FIG. 1 with parts disassembled according to the present disclosure;

FIG. 14 is a top view of a connection hub of the microwave antenna assembly of FIG. 1 according to one embodiment of the present disclosure; and

FIG. 15 is a top view of a connection hub of the microwave antenna assembly of FIG. 1 with parts disassembled according to one embodiment of the present disclosure.

DETAILED DESCRIPTION

Particular embodiments of the present disclosure will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail.

FIG. 1 shows a microwave ablation system 10 that includes a microwave antenna assembly 12 coupled to a microwave generator 14 via a flexible coaxial cable 16. The generator 14 is configured to provide microwave energy at an operational frequency from about 500 MHz to about 5000 MHz although other suitable frequencies are also contemplated.

The antenna assembly 12 includes a radiating portion 18 connected by feedline 20 (or shaft) to the cable 16. More specifically, the antenna assembly 12 is coupled to the cable 16 through a connection hub 22 having an outlet fluid port 30 and an inlet fluid port 32 that are connected in fluid communication with a sheath 38. The sheath 38 encloses radiating portion 18 and feedline 20 allowing a coolant fluid 37 to circulate from ports 30 and 32 around the antenna assembly 12. The ports 30 and 32 are also coupled to a supply pump 34 that is, in turn, coupled to a supply tank 36 via supply line 86. The supply pump 34 may be a peristaltic pump or any other suitable type. The supply tank 36 stores the coolant fluid 37 and in one embodiment, may maintain the fluid at a predetermined temperature. More specifically, the supply tank 36 may include a coolant unit that cools the returning liquid from the antenna assembly 12. In another embodiment, the coolant fluid 37 may be a gas and/or a mixture of fluid and gas.

FIG. 2 illustrates the radiating portion 18 of the antenna assembly 12 having a dipole antenna 40. The dipole antenna 40 is coupled to the feedline 20 that electrically connects antenna assembly 12 to the generator 14. As shown in FIG. 3-4, the feedline 20 includes an inner conductor 50 (e.g., wire) surrounded by an inner insulator 52, which is surrounded by an outer conductor 56 (e.g., cylindrical conducting sheath). The inner and outer conductors 50 and 56 respectively, may be constructed of copper, gold, stainless steel or other conductive metals with similar conductivity values. The metals may be plated with other materials, e.g., other conductive materials, to improve their properties, e.g., to improve conductivity or decrease energy loss, etc. In one embodiment, the feedline 20 may be formed from a coaxial semi-rigid or flexible cable having a wire with a 0.047″ outer diameter rated for 50 Ohms.

The dipole antenna 40 includes a proximal portion 42 and a distal portion 44 interconnected at a feed point 46. The distal portion 44 and the proximal portion 42 may be either balanced (e.g., of equal lengths) or unbalanced (e.g., of unequal lengths). The proximal portion 42 is formed from the inner conductor 50 and the inner insulator 52 which are extended outside the outer conductor 56, as shown best in FIG. 4. In one embodiment, in which the feedline 20 is formed from a coaxial cable, the outer conductor 56 and the inner insulator 52 may be stripped to reveal the inner conductor 50, as shown in FIG. 3.

FIG. 3 illustrates the distal portion 44 attached to the proximal portion 42. The distal portion 44 may be soldered to the inner conductor 50 of the proximal portion 42 to establish electromechanical contact therebetween. A portion of the distal end of the inner conductor 50 is inserted into the distal portion 44 such that a dipole feed gap “G” remains between the proximal and distal portions 42 and 44 at the feed point 46. The gap “G” may be from about 1 mm to about 3 mm. In one embodiment, the gap “G” may be thereafter filled with a dielectric material at the feed point 46. In another embodiment, the inner insulator 52 is extended into the feed point 46. The dielectric material may be polytetrafluoroethylene (PTFE), such as Teflon® sold by DuPont of Wilmington, Del. In another embodiment, as shown in FIG. 3, the gap “G” may be coated with a dielectric seal coating as discussed in more detail below.

With reference to FIGS. 2 and 4, the antenna assembly 12 also includes a choke 60. The choke 60 is disposed around the feedline 20 and includes an inner dielectric layer 62 and an outer conductive layer 64. The choke 60 may be a quarter-wavelength shorted choke and is shorted to the outer conductor 56 of the feedline 20 at the proximal end (not illustrated) of the choke 60 by soldering or other suitable methods. In one embodiment, the dielectric layer 62 is formed from a fluoropolymer, such as tetrafluorethylene, perfluorpropylene, and the like, and has a thickness of about 0.005 inches. The dielectric of dielectric layer 62 may extend past the choke conductor layer 64 toward the distal end of the assembly 12, as shown in FIG. 2.

Since the radiating portion 18 and the feedline 20 are in direct contact with the coolant fluid 37 these components of the assembly 12 are sealed by a protective sleeve 63 (FIG. 3) to prevent any fluid seeping therein. This may be accomplished by applying any type of melt-processable polymers using conventional injection molding and screw extrusion techniques. In one embodiment, a sleeve of fluorinated ethylene propylene (FEP) shrink wrap may be applied to the entire assembly 12, namely the feedline 20 and the radiating portion 18, as shown in FIGS. 3 and 4. The protective sleeve 63 is then heated to seal the feedline 20 and radiating portion 18. The protective sleeve 63 prevents any coolant fluid 37 from penetrating into the assembly 12. The protective sleeve 63 may be applied either prior to or after applying the outer conductive layer 64. In addition, protective sleeve 63 may also be applied at the point where the inner conductor 50 and the inner insulator 52 are extended past the outer conductor 56, thereby creating a vacuum 53 as shown in FIG. 3.

Assembly 12 also includes a tip 48 having a tapered end 24 that terminates, in one embodiment, at a pointed end 26 to allow for insertion into tissue with minimal resistance at a distal end of the radiating portion 18. In those cases where the radiating portion 18 is inserted into a pre-existing opening, tip 48 may be rounded or flat.

The tip 48, which may be formed from a variety of heat-resistant materials suitable for penetrating tissue, such as metals (e.g., stainless steel) and various thermoplastic materials, such as poletherimide, polyamide thermoplastic resins, an example of which is Ultem® sold by General Electric Co. of Fairfield, Conn. The tip 48 may be machined from various stock rods to obtain a desired shape. The tip 48 may be attached to the distal portion 44 using various adhesives, such as epoxy seal. If the tip 48 is metal, the tip 48 may be soldered to the distal portion 44.

FIG. 5 illustrates various shapes and forms of the tip 48, namely a stainless steel tip 48 a and a dielectric tip 48 b. Both tips 48 a and 48 b include an insertion base 51 having an external diameter that is smaller than diameter of the tips 48 a and 48 b allowing for easier insertion into the sheath 38. This configuration also provides for a better seal between the tip 48 and the sheath 38. The sheath 38 encloses the feedline 20, the radiating portion 18 from the tip 48 to the base 81 (FIG. 6). The sheath 38 is also secured to the base 81 of the connection hub 22 and the tip 48 such that the sheath 38 is in fluid communication with the connection hub 22 and defines a chamber 89 (FIG. 3) between the base 81 and the tip 48. The coolant fluid 37 is supplied by the pump 34 and is circulated in the chamber 89 between the radiating portion 18, the feedline 20 and the sheath 38. The sheath 38 may be any type of rigid tube, such as a catheter manufactured from polyimide and other types of polymers. The sheath 38 may be assembled by initially securing the tip 48 to the distal end of the sheath 38 and then inserting the combined sheath and tip assembly onto the assembly 12.

The assembly 12 also includes the connection hub 22, as shown in more detail in FIG. 6. The connection hub 22 includes a cable connector 79 and fluid ports 30 and 32. The connection hub 22 may include a three-branch luer type connector 72, with a first branch 74 being used to house the cable connector 79 and the second and third branches 76 and 78 to house the outlet and inlet fluid ports 30 and 32, respectively. In one embodiment, the connection hub 22 may include only the first branch 74 or two of the branches 74, 76, 78 and have the fluid ports 30 and 32 disposed directly on the first branch 74.

The connection hub 22 also includes a base 81 disposed at a distal end of the first branch 74. More than one inflow 86 and outflow 88 tube may be used. The outflow tube 88 is coupled to the second branch 76 and is in fluid communication with the bypass tube 80 through the second branch 76. In one embodiment, the assembly 12 includes one or more inflow tubes 86 a and 86 b that are fed through the third branch 78 as shown in FIG. 6.

In one embodiment, the second and third branches 76 and 78 may include various types of female and/or male luer connectors adapted to couple inflow and outflow tubes 86 and 88, respectively, from the pump 34 to the assembly 12. FIG. 7 shows the assembly 12 including two inflow tubes 86 a and 86 b. The inflow tubes 86 a and 86 b may be any type of flexible tube having an external diameter sufficient to fit inside a chamber 89 between the feedline 20 and the sheath 38. The inflow tubes 86 a and 86 b are inserted through the inlet fluid port 32. More specifically, as illustrated in FIG. 8, a female connector 102 may be coupled to the inlet port 32 either directly or to an intermediate male luer connector 104. The distal ends of the tubes 86 a and 86 b are inserted through an internal support member 103 of the female connector 102, which secures the tubes 86 a and 86 b thereto. The female and male connectors 102 and 104 allow for easy coupling of the assembly 12 to the coolant fluid system. The inflow tubes 86 a and 86 b may be secured to the third branch 78 via a glue plug 105, which may be formed by flowing glue into the third branch 78 and curing the glue via an ultraviolet source or other way known in the art.

The inflow tube 86 a is inserted into the distal end of the distal portion 44 and the inflow tube 86 b is inserted at a point proximate the midpoint of the assembly 12 (e.g., the feed point 46), as shown in FIG. 7. The inflow tubes 86 a and 86 b are then secured to the radiating portion 18 (e.g., using epoxy, glue, etc.). The inflow tubes 86 a and 86 b are positioned in this configuration to provide optimal coolant flow through the sheath 38. The fluid flow from the inflow tube 86 a is directed into the tip 48 and reflected in the proximal direction. The fluid flow from the inflow tube 86 b provides the coolant fluid 37 along the radiating portion 18. During operation, the pump 34 supplies fluid to the assembly 12 through the inflow tubes 86 a and 86 b, thereby circulating the coolant fluid 37 through the entire length of the assembly 12 including the connection hub 22. The coolant fluid 37 is then withdrawn from the first branch 74 and the second branch 76 through the outlet fluid port 30.

The above-discussed coolant system provides for circulation of dielectric coolant fluid 37 (e.g., saline, deionized water, etc.) through the entire length of the antenna assembly 12. The dielectric coolant fluid 37 removes the heat generated by the assembly 12. In addition, the dielectric coolant fluid 37 acts as a buffer for the assembly 12 and prevents near field dielectric properties of the assembly 12 from changing due to varying tissue dielectric properties. For example, as microwave energy is applied during ablation, desiccation of the tissue around the radiating portion 18 results in a drop in tissue complex permittivity by a considerable factor (e.g., about 10 times). The dielectric constant (er′) drop increases the wavelength of microwave energy in the tissue, which affects the impedance of un-buffered microwave antenna assemblies, thereby mismatching the antenna assemblies from the system impedance (e.g., impedance of the cable 16 and the generator 14). The increase in wavelength also results in a power dissipation zone which is much longer in length along the assembly 12 than in cross sectional diameter. The decrease in tissue conductivity (er″) also affects the real part of the impedance of the assembly 12. The fluid dielectric buffering according to the present disclosure also moderates the increase in wavelength of the delivered energy and drop in conductivity of the near field, thereby reducing the change in impedance of the assembly 12, allowing for a more consistent antenna-to-system impedance match and spherical power dissipation zone despite tissue behavior.

Referring to FIGS. 9 and 10, the cable connector 79 is coupled to the inner conductor 50 and outer conductor 56. More specifically, the inner conductor 50 and the inner insulator 52 extend outside the outer conductor 56 at the proximal end of the feedline 20 and the cable connector 79 is coupled to the inner and outer conductors 50 and 56. The cable connector 79 may be any type of threaded or snap connector adapted to contact the outer conductor 56 and the inner conductor 50. In one embodiment, the cable connector 79 may be an SMA type connector having an outer conductor 91, an insulator (not explicitly shown), and an inner conductor 92, which may be a hollow pin. The inner conductor 92 of the cable connector 79 fits about the inner conductor 50 and the outer conductor 91 thereof contacts the outer conductor 56, with the insulator spacing the outer and inner conductors 91 and 92 apart. Cable connector 79 may be secured to the inner and outer conductors 50 and 56 using soldering, laser welding and other suitable ways, which provide electromechanical contact therebetween at a junction point 93.

Laser welding allows coupling the cable connector 79 to the feedline 20. However, care must be exercised to avoid damaging the outer conductor 56 by the laser. Soldering avoids this issue, but at higher power levels (e.g., about 90 or more Watts) the soldering connection may begin to reflow due to the excessive heat generated by increased power. Embodiments of the present disclosure also provide for a system and method to alleviate the solder reflow by circulating a dielectric coolant fluid through the entire length of the assembly 12 up to the cable connector 79 such that the junction point 93 of the connector 79 to the inner and outer conductors 50 and 56 is cooled.

The connector 79 includes a threaded portion 94 that couples to the distal end of the cable 16, which may also have a corresponding SMA male connector. The connection hub 22 is inserted onto the distal end of the feedline 20 and is slid toward the distal end thereof. The cable connector 79 is then coupled to the proximal end of the first branch 74 thereby securing the connector hub 22 to the feedline 20 (e.g., gluing the connector hub 22 to the cable connector 79).

FIGS. 11 and 12 illustrate one embodiment wherein the first and second branches 74 and 76 are interconnected via a bypass tube 80. A beveled opening 82 is formed in the wall of the second branch 76 and is angled toward the outlet fluid port 30, as shown in FIG. 11. This configuration provides easier insertion of the bypass tube 80 into the second branch 76 as shown in FIGS. 12A-B and 13. An outlet opening 84 is also formed in the first branch 74, at approximately the proximal end thereof such that the outlet opening 84 is proximate the junction point 93 of the connector 79 and the feedline 20 allowing the coolant fluid 37 to contact the connector 79. The outlet opening 84 may be formed at any angle suitable for providing fluid flow between the first branch 74 and the second branch 76. A first end of the bypass tube 80 is attached to the outlet opening 84 such that the first end of the bypass tube 80 is proximate to the junction point 93. A second end of the bypass tube 80 is thereafter inserted through the second branch 76 and the outlet port 30 and is coupled to a male luer type connector 100, which provides for quick coupling and decoupling to the outflow tube 88, as shown in FIG. 13. The bypass tube 80 may be attached to the openings 82 and 84 using a variety of adhesives and other means suitable for sealing any gaps between the openings 82 and 84 and the bypass tube 80. The bypass tube 80 may be compression fit into the male connector 100 and/or glued thereto. The outlet port 30 is sealed via a glue plug or other means around the bypass tube 80, thereby limiting the coolant fluid 37 to outflow through the bypass tube 80. This configuration allows the coolant fluid to flow from the assembly 12 only through the opening 84.

In conventional designs, vapor pockets form at the junction between the connector 79 and the feedline 20 and prevent the coolant fluid 37 from reaching the connector 79, thereby preventing any cooling to take place. As a result, the connector 79 continues to heat up and solder attaching the coupling the connector 79 melts. The bypass tube 80 provides for unrestricted flow of the coolant fluid from the proximal end of the first branch 74 and the connector 79. The bypass tube 80 provides for flow of the coolant fluid directly from the cable connector 79 to the outlet port 30 without withdrawing fluid through the second branch 76. This configuration removes the fluid from the assembly 12 at a rate sufficient to prevent vaporization of the fluid as it comes in contact with the junction point 93 of the connector 79, thereby preventing formation of vapor pockets. In other words, the bypass tube 80 allows for the coolant fluid to circuit to the connector 79 without restrictions caused by pressure build-up resulting from the heat generated at the junction point 93.

The above-discussed coolant system provides circulation of dielectric coolant fluid 37 (e.g., saline, deionized water, etc.) through the entire length of the antenna assembly 12. In addition, the coolant is also brought in contact with the cable connector 79 allowing use of a conventional solder connection to attach the connector 79 to the feedline 20. The fluid provides cooling and enhances dielectric matching properties of the assembly 12. The coolant fluid 37 supplied to the cable connector 79 prevents solder re-flow, allowing the assembly 12 to operate at higher power levels (e.g., 150 watts). The coolant fluid 37 circulated through the sheath 38 also wicks heat away from the feedline 20, which allows delivery of high power signals to the antenna radiating section.

FIGS. 14 and 15 illustrate another embodiment of the connection hub 22 having a bifurcated outflow path configuration, in which the second branch 76 also acts as an outflow path. The connection hub 22 as shown in FIG. 14 does not include the beveled opening 82 since the bypass tube 80 is coupled to the opening 84 within the first branch 74 and is fed directly into a bifurcated coupler 106. The bifurcated coupler 106 includes a male luer connector 108 at a proximal end thereof and a bifurcated port 110 at a distal end thereof. The bifurcated port 110 includes a first port 112 and a second port 114 which are separated by a member 113 at the distal end of the bifurcated port 110 such that the first and second ports 112 and 114 then meet at a chamber 115. The first port 112 is coupled to the second branch 76 through the connector 100 and the second port 114 is coupled to bypass tube 80. This configuration provides for a dual outflow of the coolant fluid 37, from the second branch 76 and the bypass tube 80 and allows for an increased flow rate through the assembly 12.

The described embodiments of the present disclosure are intended to be illustrative rather than restrictive, and are not intended to represent every embodiment of the present disclosure. Embodiments of the present disclosure may also be implemented in a microwave monopolar antenna or other electrosurgical devices. Various modifications and variations can be made without departing from the spirit or scope of the disclosure as set forth in the following claims both literally and in equivalents recognized in law. 

1-18. (canceled)
 19. A microwave antenna assembly comprising: a hub; a cylindrical sheath extending distally from the hub, the cylindrical sheath defining a chamber; a feedline configured to deliver microwave energy, the feedline including an inner conductor, an outer conductor, and an inner insulator disposed between at least a portion of the inner conductor and the outer conductor, wherein at least a portion of the inner conductor of the feedline extends through the chamber defined by the cylindrical sheath; and a fluid flow tube extending through the chamber defined by the cylindrical sheath, the fluid flow tube configured to deliver a coolant fluid to the chamber defined by the cylindrical sheath to cool the portion of the inner conductor of the feedline extending through the chamber defined by the cylindrical sheath.
 20. The microwave antenna assembly according to claim 19, wherein the coolant fluid is a gas.
 21. The microwave antenna assembly according to claim 19, wherein the coolant fluid is a liquid.
 22. The microwave antenna assembly according to claim 19, wherein the coolant fluid is a combination of a liquid and a gas.
 23. The microwave antenna assembly according to claim 19, further comprising: a dielectric layer formed circumferentially about at least a portion of the outer conductor; and a conductive layer formed circumferentially about at least a portion of the dielectric layer, at least a portion of the conductive layer electrically coupled to a portion of the outer conductor of the feedline, wherein the dielectric layer and the conductive layer limit propagation of microwave energy in a proximal direction.
 24. The microwave antenna assembly according to claim 23, wherein the conductive layer includes a proximal end electrically coupled to the outer conductor of the feedline.
 25. The microwave antenna assembly according to claim 23, wherein the dielectric layer and the conductive layer define a quarter wavelength choke.
 26. The microwave antenna assembly according to claim 23, further comprising a second dielectric layer formed circumferentially about the dielectric layer between the dielectric layer and the conductive layer.
 27. The microwave antenna assembly according to claim 23, wherein the feedline includes a radiating section configured to deliver microwave energy to tissue and a distal end of the conductive layer is disposed proximal a proximal end of the radiating section.
 28. The microwave antenna assembly according to claim 19, wherein the fluid flow tube extends through the hub.
 29. The microwave antenna assembly according to claim 19, wherein the fluid flow tube is disposed adjacent at least a portion of the inner conductor within the chamber defined by the cylindrical sheath.
 30. The microwave antenna assembly according to claim 19, wherein the feedline includes a radiating portion configured to deliver microwave energy to tissue and wherein a distal end of the fluid flow tube is disposed proximate the radiating portion.
 31. The microwave antenna assembly according to claim 19, further comprising a tip disposed at a distal portion of the microwave antenna assembly, wherein coolant fluid flow from the fluid inflow tube is directed into the tip and reflected in a proximal direction.
 32. The microwave antenna assembly according to claim 19, further comprising a second fluid flow tube configured to deliver coolant, a distal end of the second flow tube disposed distal a distal end of the fluid flow tube.
 33. The microwave antenna assembly according to claim 19, wherein coolant flows in a first direction when flowing along an interior surface of the fluid flow tube and a second direction when flowing along an exterior surface of the fluid flow tube.
 34. The microwave antenna assembly according to claim 19, wherein the cylindrical sheath is a conducting cylindrical sheath formed of a conductive material. 